An interfacial model was proposed for the ternary thermoplastics (matrix)/elastomer/rigid-particle filler composite with high strength, high toughness, and high modulus. A dispersed phase of rigid particle as a core and elastomer as a shell that has a good interfacial adhesion with the matrix is the key point of the model. A composite with high strength, high toughness, and high modulus was obtained in the styrene (ST) and maleic anhydride (MAH) modified high-density polyethylene (HDPE)/ethylene-propylene-diene monomer (EPDM) rubber/carbon black (CB) with ditertiary butyl peroxide (DTBP) as the initiator through the reactive extrusion. The electrical resistivity measurement showed that CB of the unmodified composites distributed at the interface of the HDPE and EPDM, while that of the modified composites distributed mainly in the EPDM phase. The morphology of the ternary composite was consistent with the wetting coefficient analysis. That the mechanical properties of the gamma-ray-irradiated unmodified composites were not as good as those of the modified composites further indicated that the mechanical properties of the composite could not be improved significantly purely by introducing the interfacial adhesion and matrix crosslinking without forming the proposed dispersed phase structure. SEM observation supported the conclusion that the different phase structures are the major reason that leads to the different toughness.